The liquid rocket engine pipe is mostly welded, which makes it easy to fail under the residual stress and strain after welding. This paper establishes a numerical welding model for liquid rocket engine pipe in order to accurately predict the welding temperature field and structural field, so as to investigate the method of improving the welding quality and mitigating structural failure. The model uses a Gaussian moving hemispherical heat source, which is embedded in the 3-D finite element thermal elastic-plastic coupling analysis through a subprogram. The transient welding temperature, stress and strain distributions are calculated considering the phases of preheating, welding and cooling. Then, the temperatures and residual stresses calculated by the simulation are compared with those of the experiment. It is shown that the numerical model is accurate. Based on the verified welding model, this paper analyses the influence of welding parameters. The results indicate that increasing the heat source power is beneficial to reducing the residual stress on the premise of ensuring the welding quality.